rmoorman / elixir-monorepo-test

Example repository for demonstrating dep switching based on env for libraries

This repository contains code that demonstrates how one could handle library dependencies based on an environment variable during library development.

The idea is, that during development of the library, mix is instructed to use a local path rather than the dependency from hex, which is useful in order to develop the library locally in isolation.

What it comes down to

Within the package you want to switch a dependency based on env, make something like this appear in the mix.exs file:

    defp deps() do
        [
            my_dep(), # <- call to a helper function that resolves the dep spec
        ]
    end

    # Helper function that will resolve the dep spec for `my_dep` based on
    # a preferrably uniquely named environment variable
    defp my_dep() do
        case System.get_env("UNIQUELY_NAMED_ENV_VAR_FOR_DEV_PURPOSES") do
            "LOCAL" -> {:my_dep, path: "../my_dep"}
            _ -> {:my_dep, "~> 0.1"}
        end
    end

After that, just take care that you only set that env var only when you need to actually develop on the package itself. In order to simplify managing that environment variable throughout development, you could opt for using direnv and add a .envrc like that:

export UNIQUELY_NAMED_ENV_VAR_FOR_DEV_PURPOSES=LOCAL

When direnv is set up correctly, and you allowed it to use the directory, you might end up with the env var being correctly set whenever you enter the package's directory for local development.

What can be found in this repo then?

There are three packages that were generated using mix new [package name] with minimal adjustments:

• package/foo: core foo; would have common modules used by foo_ecto
• package/foo_ecto: a companion package to foo; it depends on foo and is the package that library users (in our example here) would install to use foo integrated with ecto
• my_app: example package that requires foo_ecto

The dependency switching logic can be found in package/foo_ecto/mix.exs.

The env variable we switch based on is FOO_DEV_DEP which seems specific enough for the example to not lead to collisions in our case.

There is also a package/foo_ecto/.envrc that sets the correct env var through direnv when entering the directory (with the option of setting an override using a local .env)

Some examples out in the wild

Oftentimes, a characteristic of a useful pattern is that it can be found elsewhere in one way or the other. See a few examples below:

• wojtekmach mentioned the pattern on the forum

• ecto_sql uses it to explicitly set dep path based on env

• various ex_aws packages use it to use a predefined local path